WASHINGTON — U.S. air power options in Iraq are limited now, because the closest aircraft that could wage a bombing campaign against Islamist militants who have captured several of the nation's largest cities are at least 800 miles away, a top defense official told USA Today Friday.

The insurgents — the Islamic State of Iraq and Levant (ISIL), an al-Qaeda splinter group — started their offensive Monday. They are trying to create an Islamic territory including Iraq and Syria. They have taken Mosul, Iraq's second-largest city, and Tikrit, the birthplace of former Iraqi dictator Saddam Hussein.

To be effective, air strikes would require the Iraqi army to step forward and engage insurgent forces on the ground, said the official, who spoke on the condition of anonymity because the details of potential air power options are not authorized to be released publicly.

In any event, U.S. warplanes are not based close enough at the moment to conduct such a campaign, he said.

"We have basically left Iraq," the official said.

The insurgents are about 100 miles from the Iraqi capital of Baghdad. The Iraqi military under Prime Minister Nouri al-Maliki has so far been unable to stop them.

Despite the limitations, Secretary of State John Kerry said Friday that President Obama will act quickly.

"Given the gravity of the situation, I would anticipate timely decisions from the president regarding the challenge ... I am confident the US will move rapidly and effectively to join with our allies in dealing with this challenge," Kerry said in London.

White House officials said they are assessing their options, both militarily and legally.

Obama "is considering all options in response to the question about potential direct action by the United States military," said White House spokesman Jay Carney. "But we would have to get back to you on how that would proceed if that decision were made."

The president said Thursday that "I think it's fair to say that in our consultations with the Iraqis there will be some short-term, immediate things that need to be done militarily, and our national security team is looking at all the options."